1. Field of the Invention
This invention relates to a DC-DC converter, specifically to a DC-DC converter provided with an overcurrent protection function.
2. Description of the Related Art
FIG. 3 is a circuit diagram showing a step-down DC-DC converter according to a conventional art, which converts an input voltage Vin to an output voltage Vout that is lower than the input voltage Vin. That is, when the DC input voltage Vin is inputted from an input power supply, the input voltage Vin is stepped down by turning on/off of an output switching device 101 made of a MOS transistor and smoothed into the DC output voltage Vout by a smoothing circuit composed of an inductor 102, a diode 103 and a capacitor 104.
To describe more concretely, when a signal S1 is applied from a controller circuit 109 to a gate of the output switching device 101, the output switching device 101 is turned on and an output current I101 flows through the output switching device 101. And the output current I101 flows through an output resistor 105 to raise the output voltage Vout linearly, while accumulating magnetic energy in the inductor 102.
When the signal S1 is not applied to the gate of the output switching device 101, although the output switching device 101 is turned off and the output current I101 does not flow, the magnetic energy stored in the inductor 102 is released as an inductor current I102, which is provided to the output resistor 105 flowing through a diode 103, while decreasing linearly to reduce the output voltage Vout.
This operation is repeated and a level of the output voltage Vout takes a triangular waveform that alternates between the increase and the decrease. In practice, the output voltage Vout is outputted as an approximately constant DC voltage, since a difference between a maximum value and a minimum value of the triangular waveform of the voltage level is negligibly small. The maximum value and the minimum value of the voltage level are controlled by the input voltage Vin and a duty ratio of the signal S1. Thus, the output voltage Vout is variably controlled by switching operation of the output switching device 101.
Further description related to the technologies mentioned above can be found in Japanese Patent Application Publication No. 2001-157442, for example.
The DC-DC converter generally requires an overcurrent protection function so that the output switching device 101 is not destroyed because of an excessive current caused by some reason while the output switching device 101 is turned on.
Regarding the above, a detection resistor 108 is interposed between the inductor 102 and the output resistor 105 and the overcurrent is detected based on an electric potential difference between both ends of the detection resistor 108 in the DC-DC converter according to the conventional art. When the overcurrent is detected, the control circuit 109 is instructed not to output the signal S1 so that the overcurrent does not flow through the output switching device 101.
However, because the detection resistor 108 is placed in a main current path, the whole output current I101 flows through the detection resistor to cause a large power loss. As a result, there arises a need for a countermeasure of some kind against the heat generation, which causes a problem in a size and cost of the DC-DC converter.